1. Field of the Invention
The subject invention relates to an assembly for sensing moisture on a sheet of glass and, more particularly, to a rain sensor for detecting rain on the windshield of an automotive vehicle to turn on the wipers.
2. Description of the Prior Art
The sensing of rain or moisture on the windshield of a vehicle for automatically controlling wiping systems has become a popular driver convenience item in the last several years. With such automation, drivers can be more focused on the road than on the manual setting of the windshield wiper controls. Traditional wiper controls are typically preset to one of several speeds, and/or the wipe rate interval is set according to the rainfall rate. Of course as the rainfall (rate) increases or decreases the proper setting to maintain a nominally clear windshield varies, and, therefore, necessitates frequent driver adjustment.
Such fully automatic, rain sensing, wiper control systems continually monitor the rain that is falling on the windshield, and respond by adjusting the wiping interval to match the rain-fall rate.
Several methods have been used to achieve such automatic wiper control and, for the most part, such methods differ only in the rain sensing detectors that are employed. That is, the rain-sensing element is generally adapted to work with or control existing interval (timed) wiping systems. The most common rain or moisture sensing detectors have been based on either the optical or the electrical properties of water. These include the prior art devices shown in U.S. Pat. No. 5,923,027 to Stam, et al, U.S. Pat. No. 6,020,704 to Buschur and in applicants copending applications set forth above.
The moisture variants that must be detected include water film, drops, mist, fog, snow, ice, etc. the most important aspect of any moisture detection system is the accuracy in the detection of the moisture. In other words, the system needs to detect moisture on the windshield while being resistant to false triggering by spurious signals, i.e., non-moisture, lights and other objects.
The subject invention provides an assembly for sensing moisture on the exterior surface of a sheet of glass comprising a first image sensor for sensing a first image of the exterior surface of the glass along a first optical axis and a second image sensor for sensing a second image of the exterior surface of the glass along a second optical axis which intersects the first optical axis at an intersection disposed adjacent the exterior surface of the glass. An image correlator is included for correlating the first and second images and producing an activation signal in response to coincidence of moisture in the respective correlated first and second images.
The invention also includes a method for sensing moisture on the exterior surface of a sheet of glass comprising the steps of viewing the exterior surface of the glass along a first optical axis to produce a first image, viewing the exterior surface of the glass along a second optical axis which intersects the first optical axis at an intersection disposed adjacent the exterior surface of the glass to produce a second image, correlating the first and second images, and producing an activation signal in response to coincidence of moisture in the respective correlated first and second images.
Accordingly, the invention addresses the rain or moisture selectivity problem by critically limiting activation of the wiper system to essentially the exterior surface of the windshield only. In other words, artifacts or extraneous, light sources or other objects that appear outside the monitored or in a focus zone on the exterior surface of the windshield are disregarded by the subject invention.